Chemical compositions of catfish and Frigate mackerel muscles were characterized. Characteristics, chemical and physical compositions were different among species as well as muscle types. Frigate mackerel muscle contained a greater content of protein compared with catfish muscle (p<0.05). Catfish, particularly ordinary muscle composed of a higher lipid content than Frigate mackerel muscle (p<0.05). From muscle protein fractionation, myofibrillar proteins were major proteins found in both muscles. Frigate mackerel muscle, both dark and ordinary muscles, contained greater contents of myofibrillar proteins than catfish muscle (p<0.05). Myosin heavy chain and actin were predominant proteins found in myofibrillar protein fraction in both fish. Dark muscle from Frigate mackerel composed of the highest sarcoplasmic proteins, especially myoglobin (p<0.05). The highest carotenoid content was noticeable in both dark and ordinary muscles from catfish muscle. Ordinary muscle from Frigate mackerel had the highest phospholipid content. Both dark and ordinary muscles from Frigate mackerel contained a greater content of sodium chloride compared with those from catfish (p<0.05). The highest contents of iron, copper and selenium were found in Frigate mackerel dark muscle (p<0.05). The pH of ordinary muscle from both species was higher than dark muscle (p<0.05). Frigate mackerel, especilly dark muscle, exhibited the most darkred color as shown by the lowest L* and b* values with the highest a* value and redness index (a*/b*) (p<0.05). Lipid from dark muscle of both species was more susceptible to oxidation than that from ordinary muscle as indicated by the lowest induction time analysed by Rancimant test. Changes in lipid, myoglobin, color and odor in dark, ordinary and whole muscles from catfish and Frigate mackerel during refrigerated storage (4C) for 15 days were investigated. A negligible change in pH was found in both species during storage. Total volatile base (TVB) contents of all muscles tended to increase with increasing storage time. Changes in free fatty acid content occurred with the highest degree in all muscles from Frigate mackerel. Formation and decomposition rates of conjugated diene were different among fish species and muscle types. Dark muscle of both species tended to have the highest peroxide value (PV) throughout the storage period. Thiobarbituric acid reactive substances (TBARS) values of all types of Frigate mackerel muscles were higher than those of catfish muscles till the end of storage period indicating a higher degree of lipid oxidation in Frigate mackerel muscle. For the changes in heme iron content, a decrease was found in Frigate mackere and no marked change was found in catfish muscle. Frigate mackerel, especially dark muscle, showed a higher content of non-heme iron throughout the storage time and a slight change in nonheme iron content was found in both species during storage. The changes in oxymyoglobin and metmyoglobin contents of muscles from both species tended to follow the same trend. Both myoglobin derivatives tended to decrease during storage indicating the oxidation of oxymyoglobin to form metmyoglobin and other derivatives. A slight change in redness index was found in catfish muscle but a marked change was observed in Frigate mackerel, especially dark muscle. For the protein carbonyl content, it was noted that the rate of protein oxidation in fish muscle depended on fish species and muscle type. The prooxidative activities of ferrous myoglobin on catfish and Frigate mackerel lipid oxidations in lecithin-liposome model systems were investigated by monitoring the changes in conjugated diene and TBARS during incubation for 180 min. The results showed that ferrous myoglobin at a concentration of 0.1 mM exhibited the highest prooxidative activity towards catfish and Frigate mackerel lipids. It was also found that the prooxidative activity of ferrous myoglobin was species and temperature dependent. The highest degree of lipid oxidation was found at 25C. The pH influenced the lipid oxidation induced by ferrous myoglobin with varying degrees depending on fish species. Considering the TBARS value, a secondary lipid oxidation product derived from conjugated diene, it was stated that lipid oxidation in catfish and Frigate mackerel was found to be the highest at pH 7. For the effect of sodium chloride on the lipid oxidation catalyzed by ferrous myoglobin, the highest conjugated diene was found in catfish the presence of 2.5 % and the TBARS value of Frigate mackerel was higher than catfish at all sodium chloride concentration. In addition, the prooxidative activity ot ferrous myoglobin was enhanced by the addition of hydrogen peroxide especially at high concentration. For the inhibitory activity of EDTA towards lipid oxidation in the presence of ferrous myoglobin, it was suggested that the ability to inhibit the lipid oxidation of EDTA was governed by EDTA concentration and fish species.